The invention relates to the crystallization of sugars and especially to the treatment of sugar solutions for crystallization with the aim to purify sugar solutions from crystallization inhibitors. In connection with the present invention, the sugars are especially reducing sugars.
In the sugar industry, crystalline sugar products are especially desirable end products. The crystallization of sugars, however, is in many cases difficult due to the presence of so-called crystallization inhibitors. The crystallization inhibitors comprise various by-products formed in sugar solutions in the sugar-processing steps preceding the crystallization, such as during the hydrolysis of the raw material, during the sugar-inversion stage and during the concentration and/or evaporation stages relating to the recovery of the desired sugars.
It is generally known in the art of sugar crystallization that crystallization inhibitors disturb the crystallization of sugars by adhering to the growing sugar crystal in the crystal growth stage by covering part of the sugar crystal surface by crystallization inhibitors. The presence of crystallization inhibitors retards the crystallization process and leads to distortions in the crystal shape.
Xylose, fructose and maltose are examples of reducing sugars where the presence of crystallization inhibitors should be avoided in the crystallization stage.
Fructose is a valuable raw material in the sweets, aroma and flavoring industries. Fructose is generally prepared using starch or saccharose as the raw material.
A typical process for preparing fructose comprises hydrolysis/isomerisation of the starch/saccharose raw material to obtain a glucose/fructose syrup, separation of fructose from the glucose/fructose syrup for example by chromatography, concentration of the fructose fraction thus obtained, pH adjustment and crystallization. During these process steps, especially during the concentration step, some dimeric and oligomeric fructose is formed and also some disaccharides are remained in the fructose solution to be crystallized. The dimeric and oligomeric fructose forms disturb the crystallization of fructose.
It is known that fructose undergoes irreversible dehydration during the crystallization process to yield several forms of difructose dianhydride impurities (Handbook of Industrial Crystallization, Chapter 3: The Influence of Impurities and Solvents on Crystallization, p. 83, ed. Allan S. Myerson, Butterworth-Heinemann, Boston 1993). Since the difructose dianhydride molecule consists of two fructose moieties, it exhibits some of the chemical and structural features of the host fructose molecule. The difructose dianhydride impurities appear to incorporate into the crystal (at <1 weight-% level), thus inhibiting the subsequent adsorption and growth of fructose molecules. The resulting fructose crystal growth rates are so low that the crystallization time in fructose manufacture is often on the order of days.
Xylose is also a valuable raw material in the sweets, aroma and flavoring industries and particularly as a starting material in the production of xylitol. Xylose is formed in the hydrolysis of xylan-containing hemicellulose, for instance in sulphite pulping processes. Vegetable material rich in xylan include the wood material from various wood species, particularly hardwood, such as birch, aspen and beech, various parts of grain (such as straw and husks, particularly corn and barley husks and corn cobs and corn fibers), bagasse, coconut shells, cottonseed skins etc.
Crystallization of xylose is carried out from xylose-containing solutions of various origin and purity, for instance from sulphite pulping liquors. In addition to xylose, the spent sulphite pulping liquors contain, as typical components, lignosulphonates, sulphite cooking chemicals, xylonic acid, oligomeric sugars, dimeric sugars and monosaccharides (other than the desired xylose), and carboxylic acids, such as acetic acid, and uronic acids.
Before crystallization, it is as a rule necessary to purify the xylose-containing solution obtained as a result of the hydrolysis of cellulosic material to a required degree of purity by various methods, such as filtration to remove mechanical impurities, ultrafiltration, ion-exchange, decolouring, ion exclusion or chromatography or combinations thereof.
Xylose is produced in large amounts in pulp industry, for example in the sulphite cooking of hardwood raw material. Separation of xylose from such cooking liquors is described, for example, in U.S. Pat. No. 4,631,129 (Suomen Sokeri Oy). In this process, sulphite spent liquor is subjected to two-step chromatographic separation to form substantially purified fractions of sugars (e.g. xylose) and lignosulphonates. The first chromatographic fractionation is carried out using a resin in a divalent metal salt form, typically in a calcium salt form, and the second chromatographic fractionation is carried out using a resin in a monovalent metal salt form, such as a sodium salt form.
U.S. Pat. No. 5,637,225 (Xyrofin Oy) discloses a method for the fractionation of sulphite cooking liquor by a chromatographic simulated moving bed system comprising at least two chromatographic sectional packing material beds, where at least one fraction enriched with monosaccharides and one fraction enriched with lignosulphonates is obtained. The material in the sectional packing material beds is typically a strongly acid cation exchange resin in Ca2+ form.
U.S. Pat. No. 5,730,877 (Xyrofin Oy) discloses a method for fractionating a solution, such as a sulphite cooking liquor, by a chromatographic separation method using a system comprising at least two chromatographic sectional packing beds in different ionic forms. The material of the sectional packing bed of the first loop of the process is essentially in a divalent cation form, such as in Ca2+ form, and in the last loop essentially in a monovalent cation form, such as in Na+ form.
WO 96/27028 (Xyrofin Oy) discloses a method for the recovery of xylose by crystallization and/or precipitation from solutions having a comparatively low xylose purity, typically 30 to 60% by weight of xylose on dissolved dry solids. The xylose solution to be treated may be, for example, a concentrate chromatographically obtained from a sulphite pulping liquor.
It is also known to use membrane techniques, such as ultrafiltration to purify spent sulphite pulping liquors (e.g. Papermaking Science and Technology, Book 3: Forest Products Chemistry, p. 86, ed. Johan Gullichsen, Hannu Paulapuro and Per Stenius, Helsinki University of Technology, published in cooperation with the Finnish Paper Engineer's Association and TAPPI, Gummerus, Jyväskylä, Finland, 2000). High-molar-mass lignosulphonates can thus be separated by ultrafiltration from the low-molar-mass components, such as xylose.
It is thus known to use ultrafiltration to separate compounds having a large molar mass, such as lignosulphonates present in a sulphite spent liquor, from compounds having a small molar mass, such as xylose, whereby compounds having a large molar mass (lignosulphonates) are separated into the retentate and compounds having a small molar mass (xylose) are enriched into the permeate. Further enriching of xylose from e.g. salts is possible for example with chromatographic methods using ion exclusion.
As a final step in the recovery of xylose, xylose is then crystallized from the xylose-rich fraction obtained in the xylose separation processes described above.
Nanofiltration is a relatively new pressure-driven membrane filtration process, falling between reverse osmosis and ultrafiltration. Nanofiltration typically retains organic molecules with a molar mass greater than 300 g/mol. The most important nanofiltration membranes are composite membranes made by interfacial polymerisation. Polyether sulfone membranes, sulfonated polyether sulfone membranes, polyester membranes, polysulfone membranes, aromatic polyamide membranes, polyvinyl alcohol membranes and polypiperazine membranes are examples of widely used nanofiltration membranes. Inorganic and ceramic membranes can also be used for nanofiltration.
It is known to use nanofiltration for separating monosaccharides, such as glucose from disaccharides and higher saccharides. The starting mixture including monosaccharides, disaccharides and higher saccharides may be a starch hydrolysate, for example.
U.S. Pat. No. 5,869,297 (Archer Daniels Midland Co.) discloses a nanofiltration process for making dextrose. This process comprises nanofiltering a dextrose composition including as impurities higher saccharides, such as disaccharides and trisaccharides. A dextrose composition having a solids content of at least 99% dextrose is obtained. Crosslinked aromatic polyamide membranes have been used as nanofiltration membranes.
WO 99/28490 (Novo Nordisk AS) discloses a method for enzymatic reaction of saccharides and for nanofiltration of the enzymatically treated saccharide solution including monosaccharides, disaccharides, trisaccharides and higher saccharides. Monosaccharides are obtained in the permeate, while an oligosaccharide syrup containing disaccharides and higher saccharides is obtained in the retentate. The retentate including the disaccharides and higher saccharides is recovered. A thin film composite polysulfone membrane having a cut-off size less than 100 g/mol has been used as the nanofiltration membrane, for example.
U.S. Pat. No. 4,511,654 (UOP Inc.) relates to a process for the production of a high glucose or maltose syrup by treating a glucose/maltose-containing feedstock with an enzyme selected from amyloglucosidase and β-amylase to form a partially hydrolyzed reaction mixture, passing the resultant partially hydrolyzed reaction mixture through an ultrafiltration membrane to form a retentate and a permeate, recycling the retentate to the enzyme treatment stage, and recovering the permeate including the high glucose or maltose syrup.
U.S. Pat. No. 6,126,754 (Roquette Freres) relates to a process for the manufacture of a starch hydrolysate with a high dextrose content. In this process, a starch milk is subjected to enzymatic treatment to obtain a raw saccharified hydrolysate. The hydrolysate thus obtained is then subjected to nanofiltering to collect as the nanofiltration permeate the desired starch hydrolysate with a high dextrose content.
Maltose is a valuable raw material in the production of maltitol (α(1→4)glucosylsorbitol), which is a sugar alcohol generally used as a sweetening agent in low-caloric, dietary and low-cariogenic foods, such as confectionary products and chewing gums. Maltitol is prepared in the form of crystalline maltitol or maltitol syrup.
Maltose is produced from a starch solution, which is first enzymatically hydrolyzed into a maltose syrup. For the production of maltitol, maltose syrup is catalytically hydrogenated to maltitol, whereafter the maltitol syrup is crystallized. The maltose syrup used as the starting material for the hydrogenation and crystallization contains varying levels of undesirable impurities, especially maltotriose. Maltotriose has a tendency to make the final maltose product unstable and hygroscopic. Maltotriose may also disturb the crystallization of maltose and maltitol. Furthermore, in the hydrogenation of maltose to maltitol, maltoriose is hydrogenated to maltotritol. Maltotritol also disturbs the crystallization of maltitol. For preparing crystalline products of high purity, it is thus necessary to purify the maltose-containing syrup from maltotriose. Various methods, such as hydrolysis with enzymes, chromatography and ultrafiltration or combinations thereof have been used for the purification of maltose syrups.
An enzymatic hydrolysis method for the production of maltose has been disclosed e.g. in U.S. Pat. No. 4,408,041 (Hayashibara). Chromatographic methods for the purification of maltose have been disclosed in U.S. Pat. No. 3,817,787 (Suomen Sokeri Oy) and U.S. Pat. No. 4,487,198 (Hayashibara), for example.
U.S. Pat. No. 3,832,285 (Hayashibara) relates to a method of producing maltose with high purity using enzymatic treatment and dialysis. U.S. Pat. No. 6,346,400 B1 (Roquette Freres) relates to a process for the preparation of a maltose-rich syrup using a sequence of enzymatic treatment, molecular sieve treatment and enzymatic treatment.
Ultrafiltration for the purification of liquors containing maltose and glucose have been described e.g. in U.S. Pat. No. 4,429,122 (UOP Inc.). This U.S. Patent discloses a process for the separation of a mono- or disaccharide, such as glucose and/or maltose, from polysaccharides by passing a mixture containing monosaccharides, disaccharides and polysaccharides through an ultrafiltration membrane. Polysaccharides are retained on the ultrafiltration membrane, while monosaccharides and disaccharides are permeated through the membrane. In this process, maltose and/or glucose are separated from oligosaccharides, but not from impurities having a smaller molar mass, such as maltotriose.
U.S. Pat. No. 4,511,654 (UOP Inc.) relates to a process for the production of a high glucose or maltose syrup by treating a glucose/maltose-containing feedstock with an enzyme selected from amyloglucosidase and β-amylase to form a partially hydrolyzed reaction mixture, passing the resultant partially hydrolyzed reaction mixture through an ultrafiltration membrane to form a retentate and a permeate, recycling the retentate to the enzyme treatment stage, and recovering the permeate including the high glucose or maltose syrup. Even in this process, the resulting glucose/maltose syrup is not free from impurities, such as maltotriose.
Japanese Patent Publication JP 51098346 A (Ajinomoto K K) discloses the preparation of high purity maltose by reacting gelatinized starch with β-amylase and ultrafiltering the solution thus obtained using a semipermeable membrane having a cut-off size of 5000 to 50000 g/mol, preferably 10000 to 30000 g/mol. A highly pure maltose is obtained as the filtrate.
U.S. Pat. No. 6,344,591 B2 (Roquette Freres) relates to modified maltitol crystals and a process for their manufacture. The process comprises liquefaction of a starch slurry, saccharification of the slurry to obtain a maltose hydrolysate, filtration and demineralization of the maltose hydrolysate and hydrogenation of the maltose hydrolysate to obtain a maltitol syrup having a maltitol content greater than or equal to 87% and a maltotriitol content lower than 1% by weight. The process may comprise a molecular-sieving stage using nanofiltration.
It is also known in the art that raffinose has an inhibiting effect on the crystallization of saccharose (Handbook of Industrial Crystallization, Chapter 3: The Influence of Impurities and Solvents on Crystallization, p. 76, ed. Allan S. Myerson, Butterworth-Heinemann, Boston 1993). The crystallization of sucrose in the presence of raffinose has also been studied in Advances in Industrial Crystallization, ed. J. Garside, R. J. Davey & A. G. Jones, The Control of Crystal Morphology by Additives: Molecular Recognition, Kinetics and Technology, p. 153, Butterworth-Heinemann, Oxford 1991).
Methods of removing raffinose from saccharose solutions have been disclosed for example in U.S. Pat. No. 3,992,260 (Agency Ind. Science Techn.). In the processes described in this reference, raffinose is hydrolyzed by means of enzymes to saccharose and galactose. Other processes for removing raffinose have been disclosed for example in U.S. Pat. No. 3,767,526 and CS 194667 (Agency Ind. Science Techn.).
U.S. Pat. No. 5,061,625 (Boehringer Mannheim Gmbh) discloses the use of microorganisms (which form α-galactosidase but not invertase) for the hydrolysis of raffinose in connection with the crystallization of saccharose. U.S. Pat. Nos. 3,836,432, 4,036,694 and 3,664,927 (Hokkaido Sugar Co.) disclose methods and an apparatus for the hydrolysis of raffinose by enzymes (α-galactosidase). Hydrolysis of raffinose by α-galactosidase has also been disclosed in U.S. Pat. No. 4,376,167 (Eni Ente Naz. Idrocarb.)
U.S. Pat. Nos. 4,333,779 and 4,312,678 (UOP Inc.) disclose the separation of crystallization inhibitors, such as glucose, fructose and raffinose from saccharose by adsorbing saccharose to an adsorbent followed by desorbtion.
Enzymatic hydrolysis in connection with xylan has been studied for example by P. Biely in the article “Microbial xylanolytic systems” in Trends in Biotechnology, vol. 3, No. 11, 1995.
However, the use of nanofiltration, enzymatic hydrolysis and/or chromatography for removing crystallization inhibitors from sugar solutions comprising reducing sugars, especially monosaccharides, has not been disclosed or suggested in the state of the art.